1. Field of the Invention
The present invention relates to an apparatus for inspecting patterns with reference to design data. More particularly, the invention concerns a pattern generating apparatus for generating reference patterns at a high speed on the basis of the design data.
2. Description of the Prior Art
Circuit patterns of LSI circuits or the like are commonly depicted or plotted by a plotting apparatus referred to as a pattern generator. This type of pattern generator operates based on the observation that a circuit pattern is constituted by a set of rectangular pattern elements of various sizes, such that a reticle is provided through irradiation of the individual rectangular pattern elements one by one.
Test or inspection of the circuit pattern thus plotted on the reticle is conducted through comparison of the pattern on the reticle under test with the design pattern serving as a reference pattern, as discussed, for example, in a Japanese periodical "Electronic Materials" Sept. 1983, pp. 38-51. The design pattern are electrically generated from data for test or inspection converted from data for driving a pattern generator for making it convenient to develop video signals. More specifically, the data for inspection or test is developed in a memory as a two-dimensional pattern to thereby generate the design pattern. On the other hand, a magnified image of the rectile to be inspected is converted into electric signals with the aid of an optoelectronic conversion device, the signals being developed on a memory in the similar manner. When a difference is found between the design pattern and the circuit pattern of the rectile developed on the memories as the result of comparison, it is determined that the rectile being inspected suffers a defect.
A method of generating the design pattern has heretofore been known, as disclosed in the Preparatory Paper For 16th SICE Lecture No. 2706 entitled "Graphic Pattern Generating Method" and Japanese Patent examined Publication No. 4709/1984. It should be mentioned that the pattern generator discussed in the lecture "Graphic Pattern Generating Method" was developed by the inventors of the present application.
These hitherto known pattern generating methods start with the recognition that the circuit pattern for LSI or the like is constituted only by rectangular pattern elements (subpatterns), wherein the rectangular subpatterns are generated on a two-dimensional plane on the basis of the apex coordinates of the rectangles. Application of this technique to the inspection or test of the circuit patterns of printed circuits will encounter a technical problem to be overcome, because such circuit pattern includes circular pattern elements and circular arc pattern elements in addition to the rectangular pattern elements. For plotting, for example, a circular pattern for a printed circuit, a circular aperture is magnified or contracted to appropriate size and optically projected for manufacturing a mask. On the other hand, in the case of the circular arc and linear patterns, circular or rectangular apertures are optically projected by using a numerically controlled machine to prepare the patterns. Such being the circumstances, generation of the circular arc patterns, circular patterns and acute angular patterns peculiar to the circuit pattern for the printed circuit board through the hitherto known techniques would require an enourmous amount of rectangular pattern elements, involving an increased memory capacity as well as an enlarged hardware scale to such extent that practical realization is rendered very difficult.